1. Field of the Invention
The present invention relates to methods of and apparatus for controlling the thickness of an annular extrusion, and more particularly, to methods and apparatus for automatically controlling the uniformity and thickness of a plastic jacket extruded about an elongated article, such as, for example, a cable core.
2. Discussion of the Prior Art
Extruding a plastic sheath or jacket about an elongated metal core is a common practice in manufacturing cable. The jacket insulates the metal core and otherwise protects the cable from damage. In the manufacture of such a jacketed cable, it is desirable, for various reasons, to accurately control the thickness of the extruded jacket. The danger of a thin jacket lies in insufficient protection of the core, bare metal, or even only a jacket thickness below a predetermined minimum is not tolerable. On the other hand, a thick jacket is unnecessary and wasteful, and it also increases the manufacturing costs of the cable. It is, therefore, desirable to control the thickness of the extruded jacket within tight tolerances.
Among known devices for measuring on a continuous basis the extruded thickness of the jacket, is an ultrasonic cable jacket thickness and eccentricity monitor. It is advantageously used in conjunction with the present invention since it affords the opportunity to monitor the thickness of each of the vertically and horizontally opposed quadrants of the jacket. Such an ultrasonic monitor is disclosed in applications, Ser. No. 268,961, filed July 5, 1972, now U.S. Pat. No. 3,827,287 in the names of Boggs, et al., and Ser. No. 268,973 filed July 5, 1972 in the names of Boggs, et al., .Iadd.now U.S. Pat. No. 3,916,676, .Iaddend.assignors to the Western Electric Co., Inc.
Problems incurred in controlling the thickness have been dealt with by the prior art. For instance, an application, Ser. No. 296,505 filed Oct. 10, 1972 in the names of A. M. .[.isley, et al.,.]. .Iadd.Isley, et al., now abandoned, but published in U.S. Pat. No. 3,932,439 to Isley et al., division of said application, Ser No. 296,505, .Iaddend.assignors to the Western Electric Co., Inc., discloses an apparatus for centering the cable core with respect to an extrusion orifice, to control the eccentricity of the cable core with respect to the extruded jacket. Controlling the eccentricity of the core with respect to the jacket equalizes the jacket thickness in diametrically opposite quadrants of the jacket, and consequently permits tighter control of the jacket thickness without danger of exposing the core in one quadrant .[.whle.]. .Iadd.while .Iaddend.the jacket becomes excessively thick in the opposite quadrant.
However, the jacket thickness may also be nonuniform when the corss-sectional shape of the cable core is different from the shape of the extrusion orifice. For instance, the cable core may have an oval or elliptical cross section. If the extrusion orifice is substantially circular, then the space between the outer surface of the core and the inner surface of the extrusion orifice varies between adjacent quadrants. As a result, the extruder jacket may be thinner, for instance, in the top and bottom quadrants with respect to the left and right quadrants.
it is known in the art to vary the shape of the core by what is known as rounding rollers. Two counteracting rollers exert a pressure to two opposite sides of the core to decrease the diameter of the core in one direction and to increase the diameter of the core in the direction where no pressure is exerted. Two sets of these rounding rollers acting in directions perpendicular to each other are capable of changing the cross section of the core from an ellipse with its major axis in a first direction to an ellipse with its major axis in a direction perpendicular thereto.
However, changing the shape of the core to increase the uniformity of the jacket thickness requires constant attention of an operator. Frequently, nonuniformity of the jacket thickness is not discovered until after the thickness in one of the quadrants of the jacket has fallen below a desirable minimum value.